1. Field of the Invention
The present invention relates generally to a circuit for detecting motion of a color video signal and, more particularly, is directed to a motion detecting circuit adapted for use with a luminance/chrominance (Y/C) separating circuit which separates chrominance and luminance signal components of a color video signal.
2. DESCRIPTION OF THE PRIOR ART
In a Y/C separating circuit according to the prior art, a motion detecting circuit is used to detect motion of a color video signal and, on the basis of such detected motion, or lack thereof, either a chroma signal extracting circuit suitable for a moving picture mode or a chroma signal extracting circuit suitable for a still picture mode is selectively employed. In the Y/C separating circuit according to the prior art, the color video signal is supplied to serially-connected first and second frame memories and, for the purposes of motion detection, the color video signal is subtracted from the output signal of the first frame memory to provide a subtracted output supplied through a low-pass filter to a level detecting circuit. Such level detecting circuit compares the mentioned subtracted output with a predetermined reference level. When the level of the subtracted output is lower than the predetermined reference level, the level detecting circuit determines that the color video signal has no motion and, in response thereto, a switching circuit selects the output of the chroma signal extracting circuit suitable for the still picture mode. On the other hand, when the subtracted output applied to the level detecting circuit is higher than the predetermined reference level, the level detecting circuit determines that the color video signal contains motion and causes the switching circuit to select the output of the chroma signal extracting circuit suitable for the moving picture mode.
However, the above described motion detecting circuit according to the prior art has relatively low sensitivity for detecting motion in a color video signal representing a scene having relatively small variations of brightness, for example, if a moving object in a picture has substantially the same luminance as the background, or for detecting movement of a so-called edge-defining signal which is inherently of high frequency. Although it has been suggested to avoid the foregoing problems by lowering the reference level employed in the level detecting circuit and thereby increasing the motion detecting sensitivity, such proposal is not considered effective because then noise components in the incoming color video signal result in erroneously detected motion.